1. Technical Field
The present invention relates to a method of connecting a carbon steel shaft with an aluminum member and a product made by this method. The method of the invention is applicable, for example, to productions of rotors for a screw compressor and for a Roots blower.
2. Background Art
Various screw compressors are know in the art. A screw compressor used as a supercharger for an automobile (e.g., a Lysholm compressor) includes a first screw-shaped male rotor, a second screw-shaped female rotor engaged with the first rotor and a casing for housing the first and second rotors in a way that the first and second rotors can rotate in opposite directions. The screw compressor sucks an external air from an opening formed at one end of its casing, the sucked air is gradually compressed by the first and second rotors inside the casing in an axial direction of the casing, and the compressed air is discharged from another opening formed at the opposite end of the casing. Each rotor has its own shafts which outwardly extend from ends of the rotor, and the shafts are supported by bearings or the like. The rotor rotates as its shafts are rotated.
One example of such a male or female rotor of the screw compressor is illustrated in FIG. 7 of the accompanying drawings. A rotor 38 has a shaft 42 and a profile portion 40 fitted over the shaft 42. The profile portion 40 has threads or ridges 48 on its outer periphery. The profile portion 40 is made from an aluminum alloy and the shaft 42 is made from carbon steel. Conventionally, the profile portion 40 is shrinkage-fitted over the shaft 42 or the profile portion 40 is molded over the shaft 42 by an insert molding technique.
However, the shrinkage fitting technique requires a certain strength on the profile portion 40. Therefore, an inexpensive aluminum alloy cannot be used as the profile portion 40. In addition, a through hole 50 must be drilled in the profile portion 40 for insertion of the shaft 42. This process raises a manufacturing cost. Furthermore, the rotor 38 is relatively heavy since its major portion is constituted by the steel shaft 44.
If the insert molding process is employed to prepare a shaft-profile combined element, a linear expansion coefficient of a center portion 44 of the shaft 42 which is positioned in the aluminum profile 40 should conform with that of the aluminum member 40 whereas a linear expansion coefficient of ends 46 of the shaft 42 on which steel parts such as a bearing and a gear are mounted should conform with that of the steel member. In short, the center portion 44 and ends 46 of the shaft 42 should be made from different materials. Furthermore, the rotor 38 is relatively heavy since it is mostly constituted by the steel shaft 44.